1. Field of the Invention
The present invention relates generally to electrostatic deposition (ESD) of aerosol particles and more particularly to an apparatus useful for ESD and methods of depositing the aerosol particles onto a substrate.
2. Technical Background
Over the years, there has been rapid progress in the areas of electronics, materials science, and nanoscale technologies resulting in, for example, smaller devices in electronics, advances in fiber manufacturing and new applications in the biotechnology field. The ability to generate and collect increasingly smaller, cleaner and more uniform particles is necessary in order to foster technological advances in areas which utilize small particulate matter. The development of new, efficient and adaptable ways of producing small particulate matter and subsequently collecting or depositing the small particulate matter onto a substrate becomes more and more advantageous.
The size of a particle often affects the physical and chemical properties of the particle or compound comprising the particle. For example, optical, mechanical, biochemical and catalytic properties often change when a particle has cross-sectional dimensions smaller than 200 nanometers (nm). When particle sizes are reduced to smaller than 200 nm, these smaller particles of an element or a compound often display properties that are quite different from those of larger particles of the same element or compound. For example, a material that is catalytically inactive in the macroscale can behave as a very efficient catalyst when in the form of nanoparticles.
The aforementioned particle properties are important in many technology areas. For example, in optical fiber manufacturing, the generation of substantially pure silica and germanium soot particles from impure precursors in a particular size range (about 5-300 nm) has been key in providing optical preforms capable of producing high purity optical fiber. Also, in the field of pharmaceuticals, the generation of particles having certain predetermined properties is advantageous in order to optimize, for example, in vivo delivery, bioavailability, stability of the pharmaceutical and physiological compatibility. The optical, mechanical, biochemical and catalytic properties of particles are closely related to the size of the particles and the size of the compounds comprising the particles. Gas-phase methods of particle generation are attractive, since gas-phase methods typically yield large quantities of high purity particles which are within a desirable size range.
Particle generators such as aerosol reactors have been developed for gas-phase nanoparticle synthesis. Examples of these aerosol reactors include flame reactors, tubular furnace reactors, plasma reactors, and reactors using gas-condensation methods, laser ablation methods, and spray pyrolysis methods.
In particular, flame spray pyrolysis has proven adept for soot particle generation for silica preform production in optical fiber manufacturing, for example, those described in commonly owned U.S. Pat. Nos. 5,979,185 and 6,260,385, the disclosures of which are incorporated herein by reference in their entirety.
Hot wall tubular furnace reactors have also proven adept for soot particle generation for silica preform production in optical fiber manufacturing, for example, those described in commonly owned US Patent Application Publications 2004/0187525 and 2004/0206127, the disclosures of which are incorporated herein by reference in their entirety.
Induction particle generators are examples of hot wall tubular furnace reactors using inductive heating elements to heat the reactor walls. Examples of such induction particle generators are described in patent application Ser. No. 11/502,286, filed on Aug. 10, 2006, the disclosure of which is incorporated herein by reference in its entirety, may be used to produce a flow of aerosol containing aerosol particles dimensionally in the nanometer range.
Enhanced surface area is an enabling physical property for many applications, such as custom spotted microarrays, high display of surface area for catalysis, high display of luminescent elements and the like. Conventional methods of producing enhanced surface area, such as the method described in PCT Publication No. WO0116376A1, the disclosure of which is incorporated herein by reference in its entirety, ball milled Corning 1737™ glass particles of size range from 0.5 μm to 2 μm. These ball milled particles are sintered onto Corning 1737™ glass substrates. Deposits of nanoparticles provide optimum surface area. However, particles in this nanometer size range are difficult to produce and deposit onto a substrate.
One method of depositing aerosol particles onto a substrate currently being explored is electrostatic deposition (ESD) for various applications, for example, in electrostatic scrubbers for pollution abatement and in electro-painting. U.S. Pat. No. 4,892,579 for example, describes a process of preparing an amorphous metal alloy body from elemental metal powders by dispersing and mixing the elemental powders in the aerosol state, collecting them either electrostatically or using a cyclone and compaction using an isostatic hot-press. U.S. Pat. No. 6,923,979 describes a method for depositing particles in the micron range onto a dielectric substrate using an alternating electric field.
Another challenge of depositing aerosol particles onto a substrate is overcoming the velocity of the flow of the aerosol from the particle generator such that the electric field used in ESD can overcome the velocity of the flow of the aerosol and the aerosol particles are deposited onto the substrate.
It would be advantageous to have an apparatus and a method for electrostatically depositing nanoparticles produced by gas-phase synthesis onto a substrate where the velocity of the flow of aerosol can be controlled.